Ice Ages: Causes
There are several natural forces that together lead to an ice
age on Earth.
The answer lies in how the orbit of the Earth around the sun changes.
The average temperature on Earth depends on the Earth’s distance
from the sun. If the Earth were closer to the sun, it would be
hotter; if the Earth were further away from the sun, it would be
A Yugoslav astronomer, Milutin Milankovitch, learned how changes
in Earth’s orbit can changes in climate to cause ice ages.
He studied three types of changes in Earth’s orbit: its shape,
the tilt of the its axis, and the wobble of the its
Shape of the Earth’s
If the Earth were the only planet
orbiting the sun, its path would be circular. But there are other
planets circling the sun too. Their gravity pulls slightly on the
Earth as they pass nearby, causing the Earth’s orbit to change
by a very small amount. The Earth’s orbit changes from circular to slightly
elongate and back again about every 100,000 years.
When the orbit is circular, the distance between the sun and the
Earth stays the same. But when the orbit is slightly elongate, the
sun and Earth vary from being farther away from each other (making
the climate colder) to being closer together (making the climate
The Earth is slightly tilted - that is what gives us our seasons.
how it works. On one side of its orbit around the sun, the Earth is
tilted towards the sun. During this time, the northern hemisphere receives
more heat so has higher temperatures - it is summer.
Six months later, the Earth is on the
other side of its orbit, and the Earth is tilted away from the
sun. Now, the northern hemisphere receives less heat so it is colder -- it
However, the Earth’s tilt changes from 22° to 24° and back again
about every 40,000 years. Right now, it is tilted at 23.5°. When the
tilt is at its greatest, differences in temperatures between summer and winter
will be greatest.
Like a spinning top as it is slowing down, the Earth’s axis
wobbles in a circle every 23,000 years.
Because of this wobble, the Earth moves
just a little bit more than one complete orbit each year. So, for
example, if the Earth is in one place its orbit on, say, 1 July,
it will be just a little bit further around the orbit on 1 July
of the following year. This is called “precession”.
If Earth’s orbit is slightly
elongate, then the distance between the sun and Earth will be different
each 1 July, making the summer either cooler or warmer.
When these three changes in Earth's orbit—its shape, the tilt of
the Earth's axis, and the wobble of the Earth's axis—are combined,
they can explain why we get glacial and interglacial periods.
Glacial periods occur when the Earth's orbit is elongate, when
the Earth's axis has a low tilt, and when northern hemisphere summer
occurs at a position on the orbit farther away from the sun so
that it is cool.
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